i don't think that critics, producers, etc. influence the creation of the music, not usually. But let's face it - as the ones who control the means of distribution (be it of product or information), they can't help but influence the way the music reaches the public, which in turn can't help but influence the way that some musicians go. John Hammond definitely had an "agenda", as did Norman Granz, Albert Lion, you name them. Chuck Nessa didn't record the Art Ensemble or Warne Marsh by pulling random names out of a hat, if you know what I mean. Getting that stuff out, and getting it talked about and played, reached people, and once that happens, hey, "influence" is under way. If the neither Basie nor the AACM never recorded, how would I, in lonely Gladewater, Texas, ever have heard them? And if critics had never written about them, how would I ever even have known about them?
This isn't intrinsically evil or anything, it's just the way stuff works in our society. It's also why so many musicians have expressed interest in having their own labels. Unfortunately, the skills needed to market product and to create it seldom reside in the same individual(s), so...
What I do have a problem with is an over-simplification of "agenda". Saying that some in the non-performing end of the business have used their position of influence to promote and act out on their own socio-psychological "issues", as some revisionist writers have been claiming (and no. I don't remember names) is probably true, but it's only partially true and it's probably not even the main truth. But that's what happens when you try to build your being on corpses - the obvious is already claimed, so you gotta go poking around for the minor shit that ain't nobody found yet and holding it up like a newly found treasure.
Is it "illuminating"? Yeah, sure. But is it revalatory? Uh....I don't think so. Not usually.